With recent development of the digital signal processing technique and semiconductor technique, there is proposed a consumer digital video standard of recording a standard television signal such as an NTSC or PAL moving picture signal as a digital signal. As its application, digital video cameras formed by integrating a digital video recording/playback apparatus and image sensing apparatus are commercially available. Some digital video cameras have a still picture recording function by exploiting the feature of digital recording.
Some digital video cameras comprise a digital I/F for connecting to a computer or the like, and have a function of capturing a photographed image into a computer. Apparatuses which comprise a plurality of recording media and can select one of them in accordance with the use purpose of an image are also put into practical use.
When such apparatus is connected to a television set to play back a recorded image, the image size is defined by the digital video standard and satisfactorily corresponds to, e.g., 720×480 pixels. To transfer an image to another medium via a digital I/F, many pixels are required in terms of the image quality.
As the number of pixels of an image sensing element increases, the image sensing element must be driven at a higher frequency in order to read out pieces of information of all pixels of the image sensing element. If pieces of information of all pixels are read out, the S/N ratio decreases and the power consumption increases.
To prevent this problem, there are proposed methods of increasing the data rate of image sensing information while suppressing the driving frequency of an image sensing element low. According to one of these methods, the image sensing plane is divided into a plurality of regions, each region is equipped with an independent charge transfer unit, amplifier, and output terminal, and image sensing signals are read out in parallel with each other.
FIG. 14 shows an example of an image sensing apparatus using the above-mentioned image sensing element. In FIG. 14, the image sensing plane of an image sensing element 1400 is divided into two, right and left regions. Reference numerals 1401 and 1402 denote photoelectric conversion/vertical transfer units; 1403 and 1404, horizontal transfer units; 1405 and 1406, amplifiers; and 1407 and 1408, output terminals. The image sensing element having this structure can provide image sensing information at a data rate twice as high as the driving frequency of the image sensing element.
The drawback of this method is degradation in image quality such that, when two regions are synthesized to generate an image, a boundary line is generated by the level difference between the regions owing to the non uniformity between the characteristics of the amplifiers and external peripheral circuits in these regions.
As a method of reducing degradation in image quality caused by the non uniformity, the black levels and white levels of regions are measured in advance to obtain correction coefficients. In sensing an image, the non uniformity is corrected using the correction coefficients.
FIG. 14 shows an arrangement example of the correction circuit. An object image formed on the image sensing element 1400 by an imaging optical system (not shown) is converted into electrical signals by the image sensing element 1400. The electrical signals are output from the output terminals 1407 and 1408 in response to a driving pulse supplied from a driving timing generation circuit (not shown).
Two image signals obtained by the image sensing element 1400 are subjected to an analog signal process and A/D-converted by analog signal processors 1409 and 1410. The digital signals are supplied to black level correction circuits 1411 and 1412 and a black level difference detection circuit 1413. The black level difference detection circuit 1413 detects the difference between black levels from the two image signals, and calculates a correction coefficient.
The correction coefficient is supplied to the black level correction circuits 1411 and 1412 to correct the black level difference on the basis of the correction coefficient. Detection of the black level difference uses a signal from the optical black pixel of the image sensing element 1400. Detection and correction value calculation are executed only once in a predetermined period, and an obtained correction coefficient is stored in a memory 1420. In subsequent image sensing, the black level difference is corrected using the correction coefficient stored in the memory 1420 without performing any detection.
The signals are then supplied to white level correction circuits 1414 and 1415 and a white level difference detection circuit 1416. The white level difference detection circuit 1416 detects the difference between white levels from the two image signals, and calculates a correction coefficient. The correction coefficient is supplied to the white level correction circuits 1414 and 1415 to correct the white level difference on the basis of the correction coefficient.
In detecting the white level difference, the image sensing element 1400 is irradiated with uniform light which provides a standard white level, and an attained image signal is used. Detection and correction value calculation are executed only once in a predetermined period, and an obtained correction coefficient is stored in a memory 1421. In subsequent image sensing, the white level difference is corrected using the correction coefficient stored in the memory 1421 without performing any detection.
A frame synthesizing circuit 1417 synthesizes right and left images into one image on the basis of the signals having undergone white level correction. A camera signal processing circuit 1418 performs a γ correction process, edge correction process, color correction process, and the like. The resultant signals are output as a luminance signal and color difference signals from an output terminal 1419.
However, in the prior art, correction cannot be performed in real time because the correction coefficient is calculated only under predetermined conditions such that a standard white image is sensed. The prior art cannot cope with dynamic variations such as temperature variations or variations over time, or variations in the focusing degree of the image sensing optical system, failing to sufficiently correct the non uniformity between regions.
Also, the prior art cannot quickly cope with dynamic variations such as the shake of an image sensing apparatus, and cannot satisfactorily correct the non uniformity between regions.